Investigation of the chemistry of natural waters including but not limited to underground aquifers, and above ground lakes is usually accomplished by taking water samples and analyzing the samples in a laboratory. Many chemical species are detected and measured for various purposes. For example, waste water discharge may be monitored for the presence of hazardous chemicals including but not limited to inorganic cations and anions, for example, sulfur compounds and metal compounds, and organic compounds. Another application is the detection and quantification of chemical tracers which are used for determining flow patterns. Chemical tracers include but are not limited to bromides, chlorides, sulfides, and pH. In addition, chemical analyses are routinely performed for chemical process flow stream monitoring of processes including but not limited to winemaking, electroplating, hydrometallurgy, papermaking, chemical manufacture, and many other industrial processes.
Analysis for determining the presence and amount of particular chemical species may be carried out using an electrochemical cell wherein the voltage potential is related to a difference in chemical concentration between a known reference solution and the unknown sample test solution. The classic electrochemical cell is two beakers having solutions of differing concentrations or compositions with a salt bridge in contact with both solutions and electrodes in each solution which are connected to a voltmeter. The classical electrochemical cell has the attributes of (1) physical isolation of the two solutions, and (2) electrical communication (via a salt bridge) between the two solutions, and is useable in a laboratory setting, but is inconvenient for field applications, especially submerged in-situ measurement applications.
Various methods and devices relying on the physical principles of the classic electrochemical cell have been used in field applications. In field applications, the beakers of the classic cell are replaced with a sensing half-cell and a reference half-cell which are placed in a test solution and connected by a voltmeter. Reference half-cells for field applications have the same two attributes of solution isolation with electrical communication as the classical electrochemical cell. Isolation of the solutions is fundamentally necessary because intermingling of solutions would change the electrochemical potential. Reference half-cells for field applications, grouped according to how solution intermingling is prevented, tend to be of two main types; non-flowing and flowing, wherein the reference solution either flows from the half-cell or it remains within the half-cell.
Electrochemical reference half-cells of the non-flowing type include the liquid filled classical cell and a gel filled submersible cell, for example, a Model 13-620-259 gel-filled calomel reference half-cell, manufactured by Fisher Scientific Company, Pittsburgh, Pa. In gel-filled cells, an amount of reference solution is placed within a vessel and remains within the vessel, hence the solution is non-flowing. In the classic cell, the connection between the non-flowing reference solution and the test solution is a salt bridge, and in the submersible cell, it is a virtually non-porous solid which closes one end of the vessel containing the reference solution. Hence, as in the classical cell, the gel-filled cell reference solution gel is physically prevented from intermingling with test solution yet is in electrical contact through the virtually non-porous solid. The virtually non-porous solid forms an electrical junction. Ideally, it is desirable to minimize the effect of such a junction on the operation of the half-cell. The effect of the junction is minimized by making the virtually non-porous solid as short or thin as practical. The classic cell cannot be submerged since the open beakers would not prevent entry of fluid or liquid in which the cell is submerged, thereby spoiling the concentration of the reference solution liquid within the cell. The submersible cell is completely filled with an incompressible gel so that when the cell is submerged, fluid or liquid cannot enter the cell. However, the virtually non-porous solid causes variable junction potential and an electrically noisy signal thereby limiting the accuracy of measurements made using this device.
Electrochemical reference half-cells of the flowing type include, for example, a Model 13-620-216 Ag/AgCl reference half-cell, manufactured by Fisher Scientific Company, Pittsburgh, Pa. Half-cells of this type require reference solution to flow or leak into the test solution. The flow is controlled by a porous or fritted opening that allows reference fluid to flow from the vessel. As in the gel-filled half-cell, the frit creates an electrical junction. The behavior of the electrical junction is stabilized by allowing the reference solution fluid to flow from the vessel into the test solution fluid. Therefore, the flowing half-cell maintains a more constant voltage potential compared to the non-flowing gel-filled half-cell. However, with a flowing half-cell, one has a tradeoff between making measurements only during the time (often limited to several hours) that there is sufficient reference solution fluid in the half-cell, or periodically adding sufficient reference solution fluid to allow longer term measurements. Moreover, the flowing half-cell is not submersible because the flow would cease or reverse thereby diluting the reference solution fluid with test solution fluid within the vessel.
A flowing, capillary type half-cell reduces the amount of reference solution liquid needed to provide a stable, constant voltage potential, as compared to a flowing non-capillary half-cell. A flowing, capillary type, for example, a Hach One model 44250 single junction reference half-cell, manufactured by Hach Chemical Co., Loveland, Colo., is fundamentally different from the non-capillary half-cells in that the end of the capillary is open rather than closed with a non-porous material or frit. Nevertheless, an electrical junction is formed by the interface between the two fluids, specifically liquids. This liquid junction is ideal because there is no plug material thereby producing a very stable signal. An electrode is mounted within the capillary and near the liquid junction close to the open end of the capillary. The reference solution liquid is in a syringe connected to the capillary tube. The capillary reduces the volume of reference solution liquid needed to flow into the test solution liquid and thereby maintain a stable voltage potential. In operation, the syringe is depressed a small amount to discharge reference solution liquid from the open end of the capillary prior to making a measurement.
Since the capillary is open, intermingling of the test solution liquid and the reference solution liquid within the capillary will eventually change the concentration of reference solution liquid at the electrode and require an additional discharge of reference fluid. Although this capillary reference half-cell has the advantage of stability, and it is convenient for benchtop measurements because the reference solution liquid is easily replenished, the ease of replenishment does not permit submerged operation and the proximity of the electrode to the open end does not permit prolonged operation because it requires frequent flow of reference solution liquid.
There is yet another reference half-cell described in U.S. Pat. No. 3,705,089 to Grubb that is gel-filled but open ended. However, the gel that is in direct contact with test solution liquid changes as a result of that contact thereby affecting the electrical potential of the half-cell. Grubb identifies the need to "renew" the gel/test solution liquid junction. In this case, the liquid junction is formed by an interface between the reference solution gel and a test solution liquid wherein the interface is distinct and the reference solution, being a gel, does not flow through the tube for operation of the half-cell. Grubb does not describe how the interface or junction degrades, but clearly indicates that renewal is necessary. Renewal of the junction is accomplished by cutting off a small segment of the gel-filled tube at its open end. Since cutting and removing material is, in general terms, a bulk volume discharge, this half-cell may be considered of the "flowing" type. A disadvantage of this half-cell is the need to renew the junction by cutting thereby limiting both the time between measurements and the remoteness of measurements.
All of the reference half-cells discussed and described above are of the single junction type. In some applications, it is desirable to have a double junction reference half-cell. The flowing type cell can be used as a double junction cell by placing a first vessel having a fritted opening within a second vessel having a fritted or ground glass opening. The solution fluid in the second vessel is different from the reference solution fluid in the first vessel. The main advantage of a double junction half-cell is that the solution fluid in the outer vessel physically and chemically isolates the reference solution from the test solution fluid while maintaining electrical communication between the two solution fluids.
It is apparent from the foregoing discussion that prior to the present invention, there was no known apparatus or method providing a half-cell that did not require renewal of the liquid junction for stable electrochemical measurements. Further, before the present invention, there was no known submersible capillary half-cell, nor was there a double junction submersible capillary half-cell. It would be advantageous to have a double junction submersible reference half-cell for detecting and measuring concentrations of chemical species. Those skilled in the art would further find advantages in a reference half-cell either single or double junction that did not require replenishment of a reference solution fluid yet provided a stable voltage potential over a long time period of at least several weeks. The present invention further provides ability to make in-situ chemical measurements in real time at a substantially lower cost than by laboratory analysis of field samples.